refine/fix isimportable-logic and ensure that 'tmpdir' has a python-importable name

--HG--
branch : trunk

py/_path/local.py

@@ -802,4 +802,8 @@ def autopath(globs=None):
 
 
 def isimportable(name):
-    return name[0].isalpha() and name.isalnum()
+    if name:
+        if not (name[0].isalpha() or name[0] == '_'):
+            return False
+        name= name.replace("_", '')
+        return not name or name.isalnum()

py/_test/config.py

@@ -151,7 +151,7 @@ class Config(object):
         if not numbered:
             return basetemp.mkdir(basename)
         else:
-            return py.path.local.make_numbered_dir(prefix=basename + "-", 
+            return py.path.local.make_numbered_dir(prefix=basename,
                 keep=0, rootdir=basetemp, lock_timeout=None)
 
     def getinitialnodes(self):

testing/path/test_local.py

@@ -363,6 +363,17 @@ def test_pypkgdir_unimportable(tmpdir):
     assert subdir.ensure("xyz.py").pypkgpath() == subdir
     assert not pkg.pypkgpath() 
 
+def test_isimportable():
+    from py._path.local import isimportable
+    assert not isimportable("")
+    assert isimportable("x")
+    assert isimportable("x1")
+    assert isimportable("x_1")
+    assert isimportable("_")
+    assert isimportable("_1")
+    assert not isimportable("x-1")
+    assert not isimportable("x:1")
+
 def test_homedir():
     homedir = py.path.local._gethomedir()
     assert homedir.check(dir=1)

testing/plugin/test_pytest_tmpdir.py

@@ -5,5 +5,5 @@ def test_funcarg(testdir):
     item = testdir.getitem("def test_func(tmpdir): pass")
     p = pytest_funcarg__tmpdir(FuncargRequest(item))
     assert p.check()
-    bn = p.basename.strip("0123456789-")
+    bn = p.basename.strip("0123456789")
     assert bn.endswith("test_func")